How Old are Saturn’s Rings and Inner Moons?

The Saturn system didn’t always look as it does today. That’s what new research from scientists at SETI and the Southwest Research Institute is suggesting. We’ve known about Saturn’s majestic rings for hundreds of years, but how old are they?

“Moons are always changing their orbits. That’s inevitable,” says Matija Cuk, principal investigator at the SETI Institute. “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons. Doing so, we find that they were most likely born during the most recent two percent of the planet’s history.”

That puts Saturn’s rings and its inner moons as forming about 100 million years ago. How did scientists come up with this number?

Cuk along with Luke Dones and David Nesvorny from the Southwest Research Institute took a closer look at the orbits of Saturn’s most interior moons. Specifically, they were looking at a phenomenon known as orbital resonance. This is when a moon’s orbital period is a simple fraction of another moon’s period. Think one-quarter or one-half. During these orbital configurations, small moons can have strong impacts on other moons.

Did you know: Orbital resonance can have big impacts. In fact, the researchers believe Ithaca Chasma, the massive canyon on Tethys, was formed by tidal forces when Tethys and Dione were in an orbital resonance.

Ithaca Chasma stretches for 2,000 kilometers and is up to 5 kilometers deep in spots. That makes it one of the largest canyons in the entire solar system.

By observing today’s orbits and using computer simulations for past ones, the researchers could get a handle on just how much the orbits of these moons grew. They found the closest moons’ orbits didn’t change as much as previous models have shown. The Tethys-Dione 3:2 orbital resonance predicted by standard models probably did not occur. Meaning? The interior moons are much younger than previously thought.

The strongest piece of evidence comes from Enceladus. Researchers believe the geysers seen by NASA’s Cassini spacecraft come from tidal interactions with Saturn. And these interactions are strong. Combine the simulations with Saturn’s strong tides and researchers believe Enceladus drifted into its current orbit over about 100 million years.

“A clear implication is that either the moons are significantly younger than the planet, or that their tidal evolution must be extremely slow. As an extremely slow-evolving system is incompatible with intense tidal heating of Enceladus, we conclude that the moon’s interior to Titan are not primordial,” the researchers write.

So, what did Saturn look like more than 100 million years ago? “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the Sun,” says Cuk. “Eventually, the orbits of neighboring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.”

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